Regulation of cell proliferation by nucleocytoplasmic dynamics of postnatal and embryonic exon-II-containing MBP isoforms |
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Authors: | Hande Ozgen Nicoletta Kahya Jenny C. de Jonge Graham S.T. Smith George Harauz Dick Hoekstra Wia Baron |
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Affiliation: | 1. Department of Cell Biology, University of Groningen, University Medical Center Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands;2. Department of Molecular and Cellular Biology, University of Guelph, 50 Stone Road East, Guelph, ON N1G 2W1, Canada |
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Abstract: | The only known structural protein required for formation of myelin, produced by oligodendrocytes in the central nervous system, is myelin basic protein (MBP). This peripheral membrane protein has different developmentally-regulated isoforms, generated by alternative splicing. The isoforms are targeted to distinct subcellular locations, which is governed by the presence or absence of exon-II, although their functional expression is often less clear. Here, we investigated the role of exon-II-containing MBP isoforms and their link with cell proliferation. Live-cell imaging and FRAP analysis revealed a dynamic nucleocytoplasmic translocation of the exon-II-containing postnatal 21.5-kDa MBP isoform upon mitogenic modulation. Its nuclear export was blocked upon treatment with leptomycin B, an inhibitor of nuclear protein export. Next to the postnatal MBP isoforms, embryonic exon-II-containing MBP (e-MBP) is expressed in primary (immature) oligodendrocytes. The e-MBP isoform is exclusively present in OLN-93 cells, a rat-derived oligodendrocyte progenitor cell line, and interestingly, also in several non-CNS cell lines. As seen for postnatal MBPs, a similar nucleocytoplasmic translocation upon mitogenic modulation was observed for e-MBP. Thus, upon serum deprivation, e-MBP was excluded from the nucleus, whereas re-addition of serum re-established its nuclear localization, with a concomitant increase in proliferation. Knockdown of MBP by shRNA confirmed a role for e-MBP in OLN-93 proliferation, whereas the absence of e-MBP similarly reduced the proliferative capacity of non-CNS cell lines. Thus, exon-II-containing MBP isoforms may regulate cell proliferation via a mechanism that relies on their dynamic nuclear import and export, which is not restricted to the oligodendrocyte lineage. |
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Keywords: | BSA, bovine serum albumin BrdU, 5-Bromo-2&prime -deoxy-uridine CDK, cyclin-dependent kinase CNS, central nervous system CRM1, chromosome region maintenance 1 C/N, cytoplasm / nucleus FCS, fetal calf serum FGF, fibroblast growth factor FRAP, fluorescence recovery after photobleaching e-MBP, embryonic myelin basic protein GFP, green fluorescent protein Golli, gene in the oligodendrocyte lineage HD, high density LD, low density LMB, leptomycin B mAb, monoclonal antibody MBP, myelin basic protein NES, nuclear export signal N/C, nucleus / cytoplasm N/M, nucleus / membrane OPC, oligodendrocyte progenitor cell pAb, polyclonal antibody PBS, phosphate-buffered saline PDGF, platelet-derived growth factor PFA, paraformaldehyde PLL, poly-l-lysine RFP, red fluorescent protein RT, room temperature SF, serum-free |
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